In the utilization of IR (infrared) detectors it is necessary to cool the detector under cryogenic conditions (&lt;120K) in order for it to operate properly. Generally, heat exchange devices known as cryostats have been employed for this purpose. These devices either operate continuously or they can be used to generate an inventory of liquid which then keeps the detector cool as it evaporates.
Furthermore, for various space related applications of IR detectors, it is also possible to utilize the vacuum of space to reduce the vapor pressure over a cryogen below its normal boiling temperature even to the point where it will freeze and permit cooling of the detector below the triple point temperature of the cryogen, e.g., &lt;63.degree. K for N.sub.2 and &lt;14.degree. K for H.sub.2. In order to do this, it is necessary to form or collect the liquid cryogen in a matrix that will retain the cryogen while the pressure is reduced. The liquid boils and possibly freezes, then heat is transferred from the detector to the liquid or solid cryogen as it evaporates or sublimes. The matrix must thus be effective in transferring heat to the cryogen in the matrix.
For continuous flow cryostats this same type of matrix can be used to stabilize the temperature if the cryostat flow varies as it does with demand flow type cryostats (Ref. U.S. Pat. No. 3,828,868 by R. C. Longsworth).
A matrix of 150 mesh copper screen has been tried as a means of trapping liquid cryogen and found to be totally ineffective due to the fact that the rapid boiling of the liquid within the screens when the pressure is reduced blows most of the liquid out of the screens.
Fine wire mesh has been used as a wick in cryogenic heat pipes (2,400 wires/in.). Attempts have been made to use fine wire mesh pads of copper or gold to trap some liquid to stabilize the temperature of demand flows cryostats. These by and large have been ineffective because the boiling action blows the liquid out of the mesh pad.